What a frustrating situation this sounds like for you and kitty!! I hope that some of the suggestions above will help, I totally agree with them (they beat me to it!! LOL) I would also like to suggest that this could be caused by stress. I have heard of stressed animals licking theirselves raw, compulsively scratching, etc. Have you noticed that your cat is a very nervous type? Is he the type that runs and hides when there's a visitor? I think those types of cats are more likely to have these compulsive behaviors.
In fact, I know a cat that used to obsessively go after his tail-he made it bleed and would bite it. Not sure if the cat didn't realize that it was HIS tail. The vet put the cat on some human antidepressants, can't remember which one it was! The vet said that cats can have OCD, obsessive compulsive disorder, just like us humans can. The antidepressant had really helped the cat a lot.
Also, it is well worth looking into the possibility of your kitty being hyperthyroid. Thyroid probs are well-known to cause behavior problems.
Please ask your vet about all of the suggestions here, we don't want you to waste money either. I'd be willing to bet that these ideas will improve things for you and kitty BIG time!
ZQ
I feel so badly for you and your kitty, there is no situation to be in that is more frustrating than one of this nature. Mrs. Savas gave you some excellent advice about the diet. So many dietary allergies are caused by the grains in dry cat food, you need to try something that contains no grain and no wheat gluten. In many instances, food allergies that have baffled vets and owners for years turn out to be caused by the wheat gluten in the food. Maybe try the type of diet that PrettyKitty1 has been feeding her Abby. She has been preparing fish, specifically tilapia, for him, and the intestinal problems that have plagued this poor cat for the last couple of years disappeared literally overnight once she took him off commercial cat food.
You have, from the sounds of it, already gone to the ends of the earth to try to control this, from the medically necessary declaw to changing everything in this cats life. Please keep us posted as to what happens.
Ghilly
Aw, the poor sweetie.
Switch your cat to a hypo-alergenic, human grade food like By Nature's Organic (no wheat, no soy, no corn, no grains of any kind, avoid high sugar content if possible). Make sure you feed wet food (6 oz per day per 10 pounds of body weight is about right) as well as dry food and lots of water. Give him yucca supplement, which is a natural anti-inflammatory.
Also, try giving him a digestive enzyme like Prozyme to help him break down anything in his food that he isn't processing properly.
The above is in case this is an allergy or other dietary situation. But, in case this is stress related, try an herbal calmer - something with valerian or skullcap (these are natural and potent sedatives, stick to the recommended dosage.
You really should get him checked for hyperthyroidism. This is a very common condition that has nothing to do with neutering. It is usually easily treatable - you'd actually be lucky if it was that.
You should consider getting him checked for pancreatitis and IBS (which is a common side effect of pancreatitis). If it turns out to be pancreatitis, you will want to stick with a low protein diet which isn't easy, but should help the condition. Pancreatitis isn't always easy to diagnose, but in your cat's situation, I really would consider it a possibility. You might want to just treat him like he has it and treat for it dietarily.
Let me know if you would like more details. Good luck and keep us posted!